Leakage reducing means



qul 14, 1942. J. P. W, JR 2,289,954

LEAKAGE REDUCING MEANS v Filed Jan. 8, 1942 2 Sheets-Shet 1 INVENT OR.

y 1942. J. P. ARNDT, .QJR 2,289,954

LEAKAGE REDUCING MEANS INVENT OR.

.Jol m PA/wdz Jr.

ORNEY.

Patented July 14, 1942 John P. Arndt, Jr., Euclid, Ohio, aseignor so The Brush Development Company,0leveland, Ohio, a corporation of Ohio Application January 8, 1942, Serial No. 425,982

19 Claims.

This invention relates to a method and appsratus for improving the performance of high impedance alternating current signal generators, and it relates more particularly to a method for mitigating the efl'ects or signal current leakage in generators of the type described.

High impedance signal generators, of which the piezo-electric unit is an example, find many commercial applications, but are restricted in their usefulness by the fact that they are essentially voltage generators rather than current generators. This will be understood when it is recognized that if current is drawn from the generator, much 01' its generated voltage will be developed across its high internal impedance and little will appear at the generator terminals; On the contrary, if no current is drawn from the generator, all of its generated voltage will appear at the terminals, and will be available for use. This limitation in high impedance generators has long been recognized, and i kept in mind in designing the load circuits to which such generators are connected. Thus, if the generator is to be connected to a vacuum tube amplifier, it is customary to design the amplifier to have a high impedance input circuit. It the latter impedance is at least a high as the internal impedance of the generator, adequate signal voltage will ordinarily be obtained in the input circuit to control the amplifier. -As the input impedance is increased in relation to the impedance of the generator, more voltage becomes available for use in controlling-the amplifier. It the input impedance is made very large compared with the gen-.

erator impedanceso that very little or no current is drawn from the generator, the maximum amount of control voltage is made available. Amplifiers having extremelyhigh input impedances, approaching infinite values, have become a reality through the invention disclosed in the copending application of J. P. Arndt, Jr., Serial Number 370,773, filed December 19, 1940, now

Patent No. 2,288,600, so that it now is possible to connect high impedance generators to amplifiers which draw little or no current from the generator. A problem then became apparent and was partially solved by Walter J. Brown by his lnventlon, Leakage reducing means, Serial Number 381,169, which will issue as Patent Number 2,282,319. The problem was that the advantages or a combination high impedance generator and an amplifier which draws little or no current from the generator could not be realized to their iullest extent it the generator had stray leakage paths of finite impedance. Such leakage paths are, of course, in shunt with the generator and with the impedance of the input circuit of the amplifier; Consequently; such leakage paths act as loadcircuits which draw current from the generator, and thereby defeat the objects soughtthe limitation of the device had become current leakage across the faces of the crystal generator. The present invention is a means and method of reducing leakage paths at the faces of the generator, and finds great usefulness in combination with the Brown invention for use with the Arndt invention, Serial No. 370,773.

The advantages of the present invention are particularly useful in connection with electrostatic generators such as condenser and electret microphones, and piezoelectric units and devices. Such electrostatic devices are inherently or a capacitive nature, whereas the leakage paths are essentially resistive. The result is that the leakage currents introduce an undesired low frequency cutoff. That is, for alternating current signals within a wide range of frequencies, the internal impedance of the generator may be suificiently low compared with the resistance or the leakage path that the leakage currents do not materially reduce the voltage which is available at the generator terminals. At frequencies below this range, however, the internal impedance becomes so high that the leakage currents cause most of the generated signal voltage to be developed across the internal impedance, and very little to appear at the terminals. In consequence thereoi, the generator becomes less eflective as a generator or these lowv frequency signals. By

means of this invention, however, the eilective impedance oi the leakage paths of th generator can be raised high enough to substantially avoid their elects, with theresult thatv the generator may be worked at much lower trequencies.

The effectiveness of the present invention in connection with the electrostatic generators is illustrated by the sound cell microphone such as that disclosed in Sawyer Patent No, 2,105,010 and Williams Patent 2,126,438. Inasmuch as the output of each piezoelectric unit in the sound cell is relatively small, it is usually necessary to connect a plurality of the units in series. The series connection results, however, in a small capacity for the microphone and thus requires a high input impedance in the amplifier for satisfactory low frequency response. However, the various leakage paths in the generators in such a microphone are apt to have impedances which are lower in value than the impedance of the microphone output circuit and the amplifier input circuit. Consequently, the generator surface leakage paths may determine the cutoff frequency and defeat the advantages whichmight have been gained through use Of the high impedance input circuit. In the past, this difllculty has been overcome to some extent by connecting in parallel a plurality of such series-connected piezoelectric units. Such construction has been found to introduce acoustical errors since it increases the size of the microphone. By means of the present invention it is possible to increase the effective resistance of the generator surface leakage paths to thepoint where it is no longer necessary to resort to the use of multiple sets connected in parallel. Accordingly, the invention makes it possible to avoid many of the acoustical errors mentioned above, and to place the cutoff frequency at a lower frequency than could, in many cases, be obtained by placing a numberof sets in parallel.

The invention is also of commercial importance in connection with low frequency devices such as pressure pickups of the type disclosed I in Gravley Patent No. 2,207,539, sinceby avoiding the effects of stray leakage paths, the pickup may be made to respond to very slow fluctuations in pressure. In instruments of this type, leakage has heretofore been a serious limiting factor in obtaining low frequency response because of the cutoff frequency which is imposed on the device by the leakage.

In view of the foregoing brief description of the invention, it will be understood that it is one object of the invention to'provide a method and I the out-put of an amplifier to raise the impedance of a leakage path of a high impedance generator whose signal controls the amplifier.

These and other objects will be apparent from the following detailed description of the invention takenin connection with the accompanying drawings.

Figure 1 illustrates the fundamental principles of my invention through a symbolic representation of a. high impedance generator of the single slab piezoelectric type connected to an amplifier.

Figure 2 illustrates in detail a method of applying my invention to a generator of the single slab type.

Figure 3 illustrates symbolically my invention applied to a two-slab generator.

Figure 4 illustrates in detail a method of apgenerator having more Figure 7 symbolically represents a parallel connected generator connected to a push-pull amplifier.

Figure 8 is a plan view of a generator showing the electrodes and guard rings. 1

Figure 9 is a sectional view taken along line 99 of Figure 8, and showing details of insulation of the electrodes and guard rings.

Referring now to Figure l, a high impedance generator 40 comprising, for example, a single slab ll of piezoelectric material such asRochelle salt, and electrodes 12 and I3, supplies signal potentials to terminals I4 and I5. A conductive stray leakage path between terminals ['4 and I5 is illustrated symbolically by the resistance I6, and a suitable load such as the amplifier I1 is connected to the terminals l4, l5. From what has been said above, it will be understood that since generator l0 has a high internal impedance, it is necessary that the load, such as amplifier II, have an impedance which is maintained high relative to the impedance of the generator if best performance is to be obtained. The adverse effects upon the generator l0 which were introduced by the leakage path I6 have been materially reduced and in some cases obviated by the invention of Walter J. Brown, Patent Number 2,282,319, which maintained the effective value of resistance l6 high relative to the impedance of generator Hi. This was done by inserting a guard electrode I8 between the terminals H and I5, and maintaining the guard I8 at substantially the instantaneous potential of terminal M.

In order to obviate or reduce thesignal current leakage paths which exist across the surfaces of a piezoelectric generator, I provide the generator ill with a guard ring 2| which extends entirely. around and in -'contact with the edge of a single plate generator. The guard ring 2| intercepts the leakage path 22 which exists between the electrodes l2 and i 3, and is connected to an amplifier or an auxiliary generator to maintain the guard ring 2| at substantially the instantaneous potential of the electrode I2. When this condition is met completely, there is no potential difference between the electrode I2 and the guard ring 2|, and the result is that no current will flow between them. The effective resistance of the leakage path will then be infinite. This follows from the fundamental relationship between resistance, voltage, and current expressed by R=E/I where E'is the potential difference which exists at any instant between electrodes 12 and i3, and where I is the current flowing between electrodes I2 and i3 through the leakage path at the instant E is measured, and R is the effective resistance of the-leakage paths. When the guard ring 21 is maintained at the same alternating potential as the electrode l2, there is no potential difference tending to cause a current to flow between them. The current I is accordingly zero, and R then equals E/ O, and is infinite. If, however, at the same instant the instantaneous potential of the guard ring 2! is slightly less than the potential of electrode l2, a potential gradient exists betweenthe .age problem is much more serious than the inelectrode and the guard ring 2| and some signal current will flow. The effective resistance of the leakage path may remain high, although being of a finite value, if the current I is sufliciently small. That is, if the potential difference between the electrode|2 and guard 2| is reduced appreciably below the value which it would have if the guard 2| were disconnected from the generator Ill, then the effective resistance of the leakage path 22 will have been increased. It will be appreciated that the physical state of the leakage path, and hence its actual resistance, is not affected by the invention even though its effective resistance as seen by the generator is increased in the manner just explained. From what has been said above, it will be understood that when the potential gradient has been suitably adjusted, the signal current flowing in the path may be reduced, and the effective resistance of the path, as seen by the generator, willhave been increased. Accordingly, it is of no signifiternal current leakage problem.

It will be appreciated that in practice it is somewhat difiicult to employ a separate generator as the auxiliary source of guard voltage where the close phase relationship explained above must be maintained. This method was fully V explained in connection with Figure 1 of the cance that the path itself rexr ains conductive,

since the flow of current in the path may be controlled by the potential gradient which exists between the termini of the'path.

My invention and Walter J. Brown's invention find their maximum usefulness when used together although for many applications they may be used separately. In Figure 1 they are shown used together and reference character l8 represents guard 8 of Figure 1 of the Brown patent.

It should be remembered that generator l0 supplies an alternating current signal; the potentials of the electrodes l2 and I3 consequently fluctuate or alternate between positive values and negative values in accordance with the frequency characteristics of the signal. If the potential of guard 2| is to be held at exactly the potential of electrode l2, it is obvious that the potential of the guard must likewise fluctuate in exactly the same manner as the potential of I the terminal. In other words, the potential of guard 2| must be continuously in exact phase relationship with the potential of the electrode l2 and of the same magnitude. When this condition prevails, the two potentials will be in fully opposed phase relationship so far as current is concerned, since no signal vor leakage current will fiow between them. It will be understood that if the potentials are not in such exact phase,

relationship, guard 2| may reach the same maximum potential as the electrode but at a different instant. -Under these conditions there will be an alternating potential difference between the two points and a signal current .will fiow between them. The same result occurs if the potentials are in phase but of different magnitude. The fact, however, that a signal current flows between them does not signify that the invention is ineffective in accomplishing its objectssince the current which flows when generator II is in operation may be less than the current which flows when the guard 2| is disconnected from the generator ll. Any reduction in the leakage currents on the surface of generator accomplished through the useof potentials of opposed phase relationship represents an improvement over conditions which prevail without the invention. The signal current which fiows between electrodes is in the nature of surface leakage, and the surface guard rings which are positioned onthe surface of the crystal the leakage current. In general the surface leak- Brown patent. Although it is possible to synchronize two generators, it is generally more expedient to use an amplifier as the auxiliary generator. That is, the high impedance generator l0 may be connected to an amplifier in such manner that the generator signal controls the input of the amplifier. The output signal of the amplifier may then be used in whole or in part to supply the guard potentials. When an amplifier is used in this manner, the close phase relationships which are required in practicing the invention may be maintained quite readily.

The amplifier H to which the generator I0 is connected is of the high input impedance alternating current type, as described in myapplication, Serial Number 370,773. The signal generator ||l supplies alternating signals to its ter- J minals l4 and I5. Terminal I4 is connected to the grid 24 of the amplifier tube 25, and terminal resistor 3| and the signal developed between electrodes l2 and I3; and 'itspolarity is such as to oppose the signal between electrodes l2 and i3. Accordingly, there is little signal potential difference between the ends of the grid leak resistance 3| with the result that little current is drawn by the amplifier from the generator. The eifective impedance of the amplifier is therefore relatively high.

Figure 1 shows by way of schematic representation that signal current leakage path l6 exists between terminals i4 and I5, and signal current leakage paths 22 exist on the surface of the generator III. In order to increase the effective resistances of these leakage paths, the

guard electrode I8 is positioned between terminals, and I5, and the guard ring 2| is positioned on the surface of the generator 10 between the electrodes II and I3. In this position the guard ring intercepts all or substantially all of the current leakage on the surface of the generator. The guards l8 and 2| are connected together and to the cathode end of resistor 28. Therefore, output voltage of the amplifier is applied between termini of a portion of each leakage path. As a result, the signal voltage which exists across the balance of each leakage path represents the difference between the generator voltage and the output voltage of the amplifier, and is much smaller than the voltage which would exist across it if the amplifier output were not so applied to the paths. Therefore, the leakage current supplied by the generator is proportionately reduced, and accordingly the effective impedance of each'path as seen by the generator is proportionately increased. Leakage paths from terminal II to shield l8 and from electrode It to guard ring 2| would be in parallel with the comparatively low impedance output circuit of the amplifier and therefore have negligible effect on the action of the circuit.

It will be recognized that the output signal obtained at terminals 29, 30 must necessarily be somewhat less than the signal potential applied to the amplifier at terminals l4 and I5. Accordingly, a small signal potential difference must exist between the grid 24 and output terpotential difference less than one-tenth the This signifies, of course, that if guard ring 2!, for example, is positioned so as to divide leakage path voltage between electrodes l2 and I3.

22 into two equal parts, the effective resistance of the leakage path will have been increased more than five times.

It is a feature of the amplifier circuit of Fig. 1 that the direct current plate voltage is prevented from reaching the signal generator Ill. This is accomplished by connecting terminal I to a tap on the battery 26 having the same D. 0. potential as the grid leak 3|. This feature and other circuit arrangements for accomplishing the same result are more fully discussed in the 'J. P. Arndt, Jr., application, Serial No. 370,773.

Inasmuch as piezoelectric generators should not be subjected to biasing direct current voltages,

it will be recognized that this feature of the circuits makes them particularly useful in combination with piezoelectric generators.

Figure 2 illustrates in detail the generator I0, and shows the electrode l2 in place on the top surface thereof. The electrode |3 is positioned on the bottom of the crystal slab H, and the lead 31 engages it. Lead 36 engages (or may be integral with) the top electrode I2. The guard ring 2| is shown as engaging the edge surfaces, a little of the top' surface, and a little of the bottom surface. It is to be understood, however, that this is for reasons of convenience in assemblying the unit and that the guard ring 2| may engage only one surface so long as it intercepts a large portion of the path between the electrodes l2 and I3. The electrodes l2 and ,I3

and the guard ring 2| may be of metal foil or of Grafoil type which is a graphite surface coating. For a more complete description of Grafoil electrodes see Williams Patent, No.

A lead 38 is connected to the-guard ring 2| and adapted to be connected to the guard electrode 8, or directly to the cathode end of resistor 28. At the area where the leads 36 and 3'! leave the crystal slab II the guard ring is cut away to preventcontact from being made be tween the lead and the guard ring. Another method of effecting the same result would be to connected; that is, electrodes 43 and 45 are electrically the same and have a-common lead 46, and electrode 44 has a lead 41. A uard ring 48 is provided around the edges of the two crystal slabs in. the leakage paths between the electrodes 43 and 44 and between electrodes 44 and 45. this position current leakage on the crystal surface between the electrodes of different polarity is intercepted. A lead 50 is connected to the guard ring 48 and to the guard electrode l8. The lead 41 is connected to the terminal l4, and the lead 46 to the terminal I5. Terminals M and I5 and guard electrode l8 may be connected to an amplifier I! as is shown in Fig. 1.

Figure 4 illustrates in detail the generator 40, and shows the electrode 43 in place on the top surface thereof. The electrode 45 is positioned on the bottom or the crystal slab 42 and the electrode 44 is positioned between the slab 4| and the slab 42. A lead 5| engages the electrode 43, and a lead 46 engages the electrode 45. As the electrodes 43 and 45 are electrically the same in a parallel connected element the two leads 5| and 46 are joined and brought out as a single lead numbered for convenience 45. The guard ring 48 is positioned on the edges of both crystals slabs 4| and 42 substantially entirely around the unit. A small'opening in the guard ring 48 is made at the point where lead 41 is brought out from between the slabs 4| and 42. Around the top surface of the slab 4| and around the bottom surface of slab 42 the guard ring 48 extends unbroken. The ring 48 is therefore positioned between electrodes 43 and 44, and between electrodes 44 and 45 where it will intercept substantially all surface leakage paths. An insulator 53 is provided between the guard ring 48 and the leads 46 and 5| to prevent contact therebetween. 'As in the single plate element ID the electrodes and guard ring may be of the foil, Grafoil, or any other type.

Figure 5 illustrates a series connected generator 56 comprised of aplurality of crystal slabs 51 and 58, and electrodes 59, 68, and 6| positioned on the faces of the crystal slabs 51, 58. A lead 62 is connected to the electrode 59, anda lead 63 is connected to the electrode 6|. The electrode 60 may be in two parts, each part being connected to a slab of crystal, and is not connected to an external circuit and may be dispensed with. However, it usually is provided so that the electrostatic field need not traverse the cement used to secure the plates together. During operation the three electrodes will be at three different potentials. Accordingly, to reduce surface leakage to a minimum, two guard rings may be necessary. The first guard ring 64 may be positioned on the surface of one of the crystal faces. As illustrated it is positioned on the top face of crystal slab 51 and itextends entirely around the crystal so that it intercepts substantially all of the current leakage which tends to flow between electrode 59 and electrode 60. The second guard ring 65 is positioned on the edges of both of the crystal slabs 51 and 58 where it will intercept current leakage between electrode 60 and the guard ring 64, and current leakage between elcctrode 60 and electrode 6|. A lead 66 is connected to the first guard ring 64 and to the cathode end of resistor 28 to establish the guard ring at a potential as near as possible to the potential of electrode 59 so that leakage between them will he ate. minimum. A lead 61 is connected to the second guard ring 65 and to a resistor 69 which establishes a voltage dividing network across output terminals 28 and 88. The point at which lead 81 connects to the resistor 88 is chosen to establish at the guard ring 65 a signal potential as near to the potential of the electrode 88 as practical. There may then be leakage between guard ring 84 and guard ring 65 but it is supplied by the low impedance output of amplifier i1 and accordingly is of little consequence. Likewise, leakage between guard ring 85 and electrode 6| will be supplied by the amplifier l1. As the guard ring 85 is maintained at an instantaneous potential substantially the same as the instantaneous potential of the electrode 88 there will be substantially no tendency for leakage between these two points. As was explained in detail in the Brown patent, slight phase shift is not seriously detrimental and in some instances may be corrected by appropriate circuits.

Figure 7 illustrates a parallel connected element 15 connected to a push-pull amplifier 16.-

surface of a crystal with certain portions masked out to establish electrode 11 and guard rings 88, 8| all in one operation. Guard rings 8|, 83 are connected to the cathode end of resistor 28 of one tube at a point to establish a potential in the guard rings which is nearly the same as the potential which exists on the electrodes 11, 18 to reduce the leakage between the electrodes 11, 18 and the guard ring 8|, 83 to a relatively small value. The guard rings 88, 82 are connected to the cathode end of resistor 28 of the other tube at a point to establish a potential in the guard rings which is substantially the same as the potential which exists on the electrode 18 to reducethe leakage between the electrode 18 and the guard rings 88, 82. guard rings 8|, 88 and 88, 82 will be supplied by the amplifier.

Figures 8 and 9 illustrate in detail a-generator somewhat similar to the generator 15 and shows a manner in which the leads and guard rings may be insulated from each other. If the second guard ring 85, which is electrically the same as guard rings 88, 82 of Figure 7, extends over the side faces of the element, an opening 18 is provided in the said second guard ring and the lead 13 extends through it. Lead 18 is connected to electrode 18. An insulator 1| isposie tioned between the second guard ring 85 and the lead 68, and an insulator 12 is provided between the lead 82 and the lead 88. The insulator 12 should be broad enough to extend over both the first guard rings 8|, 88 and the second guard ring 88 (88, 82 of Fig. 7).

Figure 6 illustratesa series connected generator 81 comprised of a plurality of crystal slabs 88, 88 and a plurality of electrodes 88, 8|, and 82. Electrode 88 is connected to the grid of a first tube in the push-pull amplifier, and electrode 82 is connected to the grid of the second amplifier Leakage between between electrode 88 and 8|, and a second guard ring 8445 positioned on the crystal slab 88 around the electrode 82 where it will intercept substan tially all current leakage between electrodes 8| and 82. The guard ring 88 is connectedto the cathode end of resistor 28 for the first amplifier tube to maintain the guard ring 88 at a potential nearly that of the electrode 88, and'the guard ring 84 is connected to the cathode end or resistor 28 for the second amplifier tube to maintain the guard ring 84 at a potentialnearly that of the electrode 82. Current leakage between electrode 88 and guard 88 and between electrode 82 and guard-84 is therefore greatly reduced. The current leakage between-the guard rings 83 and 84 will be supplied by the amplifier so it will not materially affect thegenerator 81. If the resistance of the leakage path from guard ring 83 to electrode 8| is not substantiallyequal to the resistance of the leakage path from guard ring 84 to electrode 8| then a central connection 85 from electrode 8| to the junction of the two cathode load resistors 28 may be put in.

It is to be understood that after the electrodes and guard rings have been applied to the surfaces of the crystal slabs that the unit may be covered with waterproofing material to prolong its life and render it more useful. 1 Reiterating, my invention chiefly concerns itself with building into a crystal element sufiicient guard rings in accordance with the type of electrical connection and number of crystal plates and electrodes so that all surface current leakage paths are intercepted by guard rings;

and to connect the guard rings to the output of an amplifier (whose input may be controlled by the generator) to introduce, into the leakage paths signal voltage to oppose the flow of generator leakage current. v

Although I have described my invention with a certain degree of particularity, it is to be understood that the present disclosure has been made only by way of example and that numerous changes in the details of construction of the units may be made without departing from the spirit and the scope of' the invention as hereinafter claimed, and that many circuit arrangements may be used with the different units.

tube. A first guard ring 88 is Positioned on the crystal slab 88 around the electrode 88 where it will intercept substantially all currentleakage I claim as my invention: 7

1. In a signal generator, 9. body of piezoelectric material adapted to supply a signal potential and having a signal current leakage. path on its surface which conducts signal currents and thereby tends to reduce the signal available from said generator, guard means on the surface of the piezoelectric material and adapted to intercept substantially all of said leakage.

2. In a signal generator, a body of generating material, a plurality of electrodes connected to said body of material and between which a signal potential supplied by said body of material is adapted to exist, and between which there is a signal current leakage path on the surface of the body of generating material which conducts signal currents thereby tending to reduce the signal available from said electrodes, guard means on the surface of said body of generating material positioned between said electrodes and adapted to intercept substantially all of said leakage.

3. A signal generator comprising, in combination, a piezo-electric crystal plate, two electrodes on said crystal plate between which a signal potential is developed by piezoelectric action of the said plate, said crystal plate having a signal current leakage path on its surface between said two electrodes which conducts signal currents and thereby tends to reduce the signal available from said electrodes, guard means positioned on said crystal plate to intercept said leakage, and lead means connected to said two electrodes and to said guard means.

i. A piece-electric unit comprising, in combination, a piezocctive hody having a pair of faces first and second electrode means disposed one on each of said faces, guard means connected to the surface of the piezoelectric body, and lead means connected to said first and second electrode means and to said guard means and adapted to connect said unit in a circuit.

5. A parallel connected piece-electric unit comprising, in combination, a first slab of piezoactive material, a second slab of piece-active material, central electrode inears disposed between said first and said second slabs of piezoactive material and in conductive engagement with each, first outside electrode means in conductive engagement with said first slab of piezoactive material, second outside electrode means in conductive engagement with said second slab of piezo-active material, said unit having a signal current leakage path, guard means positioned on said piezo-active material and in said leakage path, first lead means connected to said outside electrode means, second lead means connected to said central electrode means, said first and second lead means being adapted to carry the current generated by said unit, and lead means connected to said guard means.

6. A series connected piece-electric unit comprising, in combination, a first slab of piezoactive material, a second slab of piece-active material, central electrode means disposed between seid first and said second slabs of piezoactive material and in conductive engagement with each, first and second outside electrode means respectively in conductive engagement with said first and said second slabs of piezoactive material, each of said slabs having a leakage path betweenits outer electrode and its innor electrode, first guard means connected to the surface of said pieso-active material and in one of said leakage paths, second guard means connected to the surface of said piece-active material and in the other of said leakage paths, lead means connected to said outside electrode means and adapted to carry the current generated by said unit, first guard lead means connected to said first guard means and second guard lead means connected to said second guard means, said first and second guard lead means being adapted to supply said guard means with alternoting potential.

7. In combination, a high impedance signal generator element adapted to supply a signal potential and having a signal current leakage path on its surface which conducts signal cur- I rents and thereby tends to reduce the signal available from said generator element, guard means on the surface of the generator element, and voltage supply means connected to the guard means to reduce the flow of generator element signal current along said surface leakage path.

8. In combination, a high impedance signal generator element adapted to supply a signal voltage at its electrodes and having a leakage path which conducts signal currents and thereby tends to reduce the signal available from said generator element; and means for reducing the loss of signal resulting from said leakage, said means comprising at least one guard electrode disposed on said generator element and in said leakage path intermediate its electrodes, and an amplifier having an input circuit and an output circuit, said amplifier having its input circuit connected to the generator element electrodes and having at least a part of the output signal of its output circuit applied between the guard electrode and an adjacent point on the leakage-path in opposed phase relationship to maintain voltage between said guard and point greater than the voltage established between them by the flow of said leakage current in said leakage path.

9. In combination, a high impedance signal generator element adapted to supply a signal voltage at its electrodes and having on its surface and in shunt with said electrodes a signal current leakage path which tends to reduce the signal available from said generator element; and means-for reducing the loss of signal resulting from said surface leakage, said means comprising a pair of spaced guard electrode means disposed on the surface of said element and in said leakage path; and an amplifier having an input circult and output circuit, said amplifier having its input circuit connected to the generator element electrodes and having at least a part of the output voltage oi its output circuit applied between said guard electrodes to maintain said guard electrodes individually at substantially the instantaneous signal potential of the generator element electrode to which it is nearest.

16. In combination, a, high impedance generator element adapted to supply an alternating current signal at its electrodes and having a sig nal current leakage path across its surface which conducts signal currents between the electrodes and thereby tendsto reduce the signal available from said generator element; and an auxiliary source of signal connected in shunt with a fractional part of the leakage path, said auxiliary source-being adapted to increase the signal-current potential-drop across said fractional part when connected in shunt therewith, and thereby to decrease the potential drops across the remaining parts of the leakage path.

11. The combination as claimed in claim 10 wherein the auxiliary source is the output of an amplifier whose input is connected to the high. impedance generator element.

12. The combination as claimed in claim 10 wherein said auxiliary source is adapted to maintain the potential drop across said fractional part of the leakage path approximately equal to the potential drop which exists between the said electrodes when the leakage current flowing between them is zero, thereby to substantially eliminate signal potential drops across themmaining parts of the leakage path.

13. The combination as claimed in claim 10 wherein said fractional part of the leakage path extends from one-electrode of the generator element to a guard electrode disposed on the surface of the generator element in said leakage pathintermediate said electrodes.

14. The combination as claimed in claim 10 wherein said fractional part of the leakage pathextends between a pair of guard electrodes disposed in said leakage path in spaced relation to each other and to the electrodes of said generator element.

15. In combination, a high impedance generator element adapted to supply an alternating current signal at its electrodes and having a signal current leakage path across its surface which conducts signal currents between said electrodes and thereby tends to reduce the signal available rrom said generator element; and an auxiliary signal connected in shunt with a fractional part of the leakage path and in opposed phase 1 between conductor means connected in a circuit,

said generator element and said circuit eachhaving a signal current leakage path which conducts signal currents thereby tending to reduce the signal available from' said generator element; first guard means on the surface of said generator element and second guard means in said conductor leakage path, an amplifier having an input circuit and an output circuit, said amplifier having its input circuit connected to the signal output of said generator element and having at least a part of the output signal of its output circuit applied to said first and second guard means to reduce the flow of generator element signal current through said leakage paths.

18. In combination, a signal generator element adapted to supply a signal voltage to a circuit, said generator element having surface current leakage and said circuit having current leakage which together tend to reduce the signal voltage supplied to said circuit, first guard means associated with said generator element and second guard means associated with said circuit, and means connected to said first and said second guard means and adapted to utilize a portion 01' the generated signal voltage for applying to said first and said secondguard means a potential to substantially reduce said surface current leakage and said current leakage in said circuit.

19. A signal generator comprising, in combination, a plate of piezoelectrically active crystal material adapted to supply a signal potential, first and second electrode means on surfaces of the piezoelectrically active crystal material, said crystal material having asignal current leakage path across its surface between said first and said second electrode means which conducts signal currents and thereby tends to reduce the signal available from the crystal material, guard means positioned on the surface of the piezoelectrically active crystal material and in the leakage path between the first and the second electrode means to intercept said leakage, first lead means connected to said first electrode means, second lead means connected to said 'second electrode means, and third lead means 

